If you do the method Sledge mentions, you could move the particles toward the camera and (if needed) shrink them a little. This way they appear in front of the object, partially obscuring it. Actually, forget about shrinking them, they will look like they are going around the object, which (if it's fire or smoke) is what they should do anyway.
To manage the collision expense, you could have one or two transparent entities at different heights that just check for collisions and then adjust all the particles accordingly, based on their height above your car. It wouldn't be guaranteed every time but it might reduce the problem.
To manage the collision expense, you could have one or two transparent entities at different heights that just check for collisions and then adjust all the particles accordingly, based on their height above your car. It wouldn't be guaranteed every time but it might reduce the problem.